1. Field of the Invention
The present invention relates to an automobile hydraulic shock absorber provided between the vehicle body and the wheels of an automobile.
2. Description of the Related Art
A conventional automobile hydraulic shock absorber is composed of a hydraulic cylinder, and a diaphragm or the like provided to the hydraulic cylinder to generate a damping force (see WO 2004/065817, for example). The hydraulic cylinder of the hydraulic shock absorber described in WO 2004/065817 is provided with a cylinder body, a piston, and a piston rod. The piston divided inside of the cylinder body into a first oil chamber at the bottom-end side, and second oil chamber at the top-end side. The piston is attached to the piston rod.
The bottom end portion of the cylinder body is attached to a vehicle wheel via a rubber cushion. The piston rod passes through the top end portion of the cylinder body to protrude above the cylinder body. The top end portion of the piston rod is attached via a rubber cushion to a shock absorber connecting portion attached to the vehicle body.
A communicating passage for communicating the first oil chamber and second oil chamber with each other is formed in the piston. A diaphragm is provided to the communicating passage so that a damping force is generated when operating oil flows in the communicating passage. In other words, a damping force is generated when the hydraulic shock absorber extends or retracts and operating oil flows through the communicating passage from one oil chamber to the other oil chamber.
In order for the hydraulic shock absorber to extend or retract, the change in volume of the first oil chamber must match the change in volume of the second oil chamber. However, since the piston rod is inserted into the second oil chamber, the change in volume of the second oil chamber is less than the change in volume of the first oil chamber. In the hydraulic shock absorber disclosed in WO 2004/065817, such a problem is overcome by providing a volume adjustment mechanism inside the cylinder body.
The volume adjustment mechanism has a structure whereby a free piston which forms a part of the wall of the first oil chamber is pushed by the pressure of a high-pressure gas. The free piston is inserted so as to be able to move inside the cylinder body. The first oil chamber is formed between the free piston and the piston attached to the piston rod. High-pressure gas is charged into the space between the free piston and the bottom end of the inside of the cylinder body. In other words, as the hydraulic shock absorber extends and retracts, the free piston moves so that the change in volume of the first oil chamber matches the change in volume of the second oil chamber.
In the hydraulic shock absorber thus provided with a volume adjustment mechanism, the pressure of high-pressure gas is continuously applied from the first oil chamber to the piston for dividing the first oil chamber from the second oil chamber. The pressure of the high-pressure gas is exerted on the entire area of a pressure-receiving surface composed of the bottom surface (surface facing the free piston) of the piston via the operating oil inside the first oil chamber. The pressure of the high-pressure gas is also transmitted from the first oil chamber to the operating oil inside the second oil chamber via the operating oil in the communicating passage of the piston. In other words, the pressure of the high-pressure gas acts on the pressure-receiving surface composed of the bottom surface of the piston, and the pressure-receiving surface composed of the top surface of the piston.
The pressure-receiving surface composed of the top surface of the piston has a surface area smaller than that of the pressure receiving surface composed of the bottom surface of the piston, by an amount equal to the cross-sectional area of the piston rod. The piston is therefore pushed by an oil pressure (pressure of the high-pressure gas) corresponding in size to the difference in surface area between the pressure-receiving surfaces, and the piston rod moves toward the top end portion of the cylinder body so as to push the piston rod out from the cylinder body. In the present specification, the force with which the piston rod is pushed due to the difference in surface area of the pressure-receiving surfaces is referred to as the gas reactive force.
When the piston rod is pushed out from the cylinder body in this manner, the hydraulic shock absorber extends, and the rubber cushion provided between the hydraulic shock absorber and the vehicle body, or between the hydraulic shock absorber and the vehicle wheel, undergoes elastic deformation and hardens. Small bumps on the road over which the vehicle wheel rolls during travel are insufficient to cause the piston of the hydraulic shock absorber to move in relation to the cylinder body, and shocks that occur in such cases cannot be dampened. Small shocks that occur when the vehicle wheel rolls over small bumps during travel must be dampened by the rubber cushion. However, when the rubber cushion hardens as described above, such small shocks are not dampened by the rubber cushion, and are transmitted to the vehicle body.
In order to overcome such problems, a compression coil spring is provided inside the cylinder body in the hydraulic shock absorber disclosed in WO 2004/065817. This compression coil spring is provided inside the cylinder body such that the piston rod passes through the compression coil spring, so that the piston is pushed toward the free piston. The length of the compression coil spring is such that the compression coil spring extends from the piston to the other end portion of the cylinder body when the piston is positioned in a normal zone.
In other words, the piston is pushed toward the free piston by the spring force of the compression coil spring, and the gas reactive force described above is thereby cancelled out.
Besides the conventional hydraulic shock absorber described in WO 2004/065817, a conventional hydraulic shock absorber in which a compression coil spring is provided inside the cylinder body is also described in Japanese Laid-open Patent Publication No. 2001-193782, for example.
The compression coil spring in the cylinder body as described in Japanese Laid-open Patent Publication No. 2001-193782 is provided in order to prevent the piston in the fully extended hydraulic cylinder from colliding with the top end portion of the cylinder body. The compression coil spring does not normally push on the piston, and pushes on the piston only when the hydraulic cylinder is markedly extended.
A space for accommodating the compression coil spring for cancelling out the gas reactive force is necessary inside the cylinder body disclosed in WO 2004/065817. The hydraulic shock absorber disclosed in WO 2004/065817 therefore has drawbacks in that the overall length thereof is increased by an amount commensurate with the required space.
The weight of the compression coil spring in the cylinder body also increases, since the compression coil spring must be formed so as to have a length greater than the gap between the piston positioned within the normal zone and the top end portion of the cylinder body.
Furthermore, the spring force of the compression coil spring and the gas reactive force for raising the piston vary depending on the stroke position of the piston. In other words, in the hydraulic shock absorber disclosed in WO 2004/065817, when the piston is extended from the normal zone, the spring force is increased and the gas reactive force is reduced by the increased stroke amount. The spring force is greater than the gas reactive force in this case.
Conversely, in a case in which the hydraulic shock absorber is retracted from the state in which the piston is in the normal zone, the spring force is reduced and the gas reactive force is increased by the increased stroke amount. The gas reactive force is greater than the spring force in this case. In other words, the hydraulic shock absorber disclosed in WO 2004/065817 must generate a predetermined damping force while being subject to the effects both of variation in the spring force and variation in the gas reactive force as described above. It is therefore difficult to set the damping force to the optimum value in this hydraulic shock absorber.
In the hydraulic shock absorber disclosed in Japanese Laid-open Patent Publication No. 2001-193782, the overall length is also increased by the accommodation of a compression coil spring inside the cylinder body, the same as in the hydraulic shock absorber described in WO 2004/065817.
In the hydraulic shock absorber disclosed in Japanese Laid-open Patent Publication No. 2001-193782, when the stroke amount is small, whether during retraction or extension, the gas reactive force cannot be cancelled out. Therefore, when a vehicle provided with this hydraulic shock absorber is traveling on a straight road or a gently curving road, shocks that occur when a vehicle wheel rolls over small bumps on the road surface are transmitted to the vehicle body via the hydraulic shock absorber and the rubber cushion, resulting in poor ride quality.